The
morphemes in the list on the left contribute in some way to the
insertion of the words in a particular grammatical context, so that
the word agrees with this context in terms of tense, number, person
etc. The changes in meaning that these morphemes bring are minimal.
These are called inflectional morphemes,
note that these are all suffixes.

The
morphemes in the list on the right bring considerable semantic
changes to the word, often word class is changed, e.g. modern (adj.)
> modernise (v.); drink (v.) > drinkable (adj.); nation (n.) >
national (adj). These are called derivational morphemes
because they are used to derive new words. Derivational morphemes may
be prefixes or suffixes.

One
important observation must be made regarding inflectional morpheme.
Consider the following examples:

cats

/kæts/

dogs

/dɒgz/

horses

/ˈhɔ:sɪz/

Note
the three different pronunciations of the plural morpheme (on the
basis of phonological characteristics of the preceding consonant).
Different surface realisations of what is essentially the same thing
(a plural marker) are known as allomorphs.

Another
example of allomorphy is in the suffix for regular verbs in the past
and past participle, -ed.

kissed

/kɪst /

pleased

/pli:zd/

decided

/dɪˈsaɪdɪd/

Another
consideration regards the inflection of a small number of very common
words in the English language. In the past simple tense, go,
rather than being *goed,
is went; the
comparative form of good
is better,
not *gooder. This
phenomenon, in which the allomorph isn’t just a phonological
variation on the basic stem, is called suppletion.

In the
next two exercises you will be expected to classify morphemes on the
basis of the features discussed in this and the preceding pages.